Rollup merge of #83040 - lcnr:unused-ct-substs, r=oli-obk

extract `ConstKind::Unevaluated` into a struct

r? `@oli-obk`

diff --cc compiler/rustc_trait_selection/src/traits/const_evaluatable.rs
Simple merge

diff --cc compiler/rustc_trait_selection/src/traits/fulfill.rs
Simple merge

diff --cc compiler/rustc_trait_selection/src/traits/select/mod.rs
Simple merge